The present invention relates generally to an amplitude shift keying (ASK) modulator in a radio-frequency identification (RFID) system and, more particularly, to a transmitter having the same in an RFID system.
RFID technology is widely used in various applications such as electronic payment, security and stock inventorying, etc. A typical RFID system includes a reader, a tag and a database platform.
ASK modulation is one of the most popular schemes to be used in RFID communication. FIG. 1A illustrates an ASK modulation scheme. Referring to FIG. 1A, a signal S1 which has an amplitude “A” may be modulated to be a signal S2 having an amplitude “B.” A critical parameter so called modulation depth in ASK modulation is defined as follows:
      Modulation    ⁢                  ⁢    depth    =                              (                      A            -            B                    )                A            ·      100        ⁢          %      .      Generally, the modulation depth may have to be above 90%, if not, there may be an electromagnetic radiation problem.
FIG. 1B illustrates another ASK modulation scheme. Referring to FIG. 1B, a signal S3 which has an amplitude “A′” may be modulated to be a signal S4 having an amplitude “B′,” while B′=0 in this example. The modulation depth in this example in accordance with the above equation is 100%, i.e. the so called on-off keying (OOK). In such case, an external tag may not work because the received signal S4 has no power to activate the tag. Accordingly, it is crucial to control the modulation depth in an RFID system.
FIG. 1C is a schematic diagram of a conventional ASK modulator. Referring to FIG. 1C, an ASK modulator 1 includes an oscillator 10, a buffer amplifier 11, a modulation means 12, a band pass filter 13, an antenna 14 and a low pass filter 15. The modulation means 12 further includes a p-intrinsic-n (PIN) diode modulator 121, a power amplification circuit 122 and a voltage-current conversion circuit 123.
The PIN diode modulator 121 may inevitably generate noise or spurious radiation components which may be amplified by the power amplification circuit 122. Therefore, the band pass filter 13 to eliminate the undesired noise or spurious radiation components is indispensable for the ASK modulator. Moreover, the low pass filter 15 may receive a digital signal from a terminal “a” and generate analog signal to control the modulation means 12, which means that the modulation means 12 may be controlled by only two voltage levels, i.e. high (VDD) and low (Ground) levels. Accordingly, the low pass filter 15 is a critical element in the ASK modulator 1. However, the use of low pass filter 15 and the band pass filter 13 may increase the cost of the ASK modulator 1.
Moreover, the PIN diode (not shown) of the PIN diode modulator 121 is an active component, which may result in noise signals. However, such undesired noise signals may not be eliminated but amplified by the power amplification circuit 122.
Furthermore, a PIN diode may not be integrated into a chip with other electronic components of an ASK modulator. FIG. 1D is a schematic diagram of a conventional ASK modulator 1′. Referring to FIG. 1D, the ASK modulator 1′ includes an oscillator 10′, a buffer amplifier 11′, a control circuit 16′, a PIN diode modulator 121′, a power amplifier 122′ and an antenna 14′. The control circuit 16′ may include a micro control unit (not shown) and/or a digital processor (not shown). The buffer amplifier 11′, the control circuit 16′ and the power amplifier 122′ may be integrated into a chip to form an integrated circuit (IC) 17′. As shown in FIG. 1D, the IC 17′ has five pins t1, t2, t3, t4 and t5, wherein pin t1 receives carrier signals from the oscillator 10′; pin t2 connects the output of the buffer amplifier 11′ to the PIN diode modulator 121′; pin t3 is used to send a control signal to the PIN diode modulator 121′; pin t4 connects the PIN diode modulator 121′ to the input of the power amplifier 122′; and pint t5 connects the output of the power amplifier 122′ to the antenna 14′. The extra pins t2 and t4 of the IC 17′ may cause more harmonics or spurious signals and also increase the package cost of the ASK modulator 1′.
It may therefore desirable to have a cost efficient modulator with a simplified circuit.